Generally, an optical sub-assembly is used for a passive optical network (PON) based on ITU-T G.983.3 standard, which covers the distribution of analog optical signals, such as CATV, as well as the bi-directional digital optical communication, such as Internet, through the optical fiber. For the optical sub-assembly, a diplexer and a triplexer are mostly being used.
For PON using two different optical wavelengths as an upstream and a downstream signal, the diplexer integrally includes a wavelength coupler, a laser diode, a digital receiver.
A system in charge of distributing the analog optical signal has three different optical signal bandwidths. Major bandwidths prescribed in ITU-T G.983.3 may include an upstream digital optical signal (1260-1360 nm) sent from a subscriber, a downstream digital optical signal (1480-1500 nm) including data and IP signal, and a downstream analog optical signal (1550-1560 nm) for distributing a video signal of, e.g., CATV.
A diplexer adapted to a conventional bi-directional communication is comprised of a laser diode (LD) for an upstream signal and a digital receiver (PIN-TIA) for receiving a downstream digital signal.
FIG. 1 is a perspective view showing a conventional integrated diplexer optical sub-assembly, FIG. 2 is a cross-section view showing the diplexer optical sub-assembly, and FIG. 3 shows a perspective view cross-sectioned from the optical sub-assembly in FIG. 2.
From the drawings, major constituents are a laser diode (LD) 11, a lens 12, a 45-degree -reflecting filter 13, a ferrule 14, and a receiver 15.
The laser diode (LD) 11 provides an optical signal, for which an approximately 1310 nm FP laser diode or a DFB laser diode may be used. The lens 12 delivers the optical signal from the LD 11 to the 45-degree-reflecting filter 13, which commonly includes an aspherical lens.
The 45-degree-reflecting filter 13 separates the upstream and downstream signals at an angle of 45°, in which the optical isolation between the signals is approximately −20 dB.
The ferrule 14 delivers the optical signal from the 45-degree-reflecting filter 13 to an optical fiber.
The receiver 15 receives the optical signal incident through the 45-degree-reflecting filter 13, which includes an analog receiver and a digital receiver. A digital receiver typically adopts InGaAs PIN-TIA.
In the above-constructed conventional optical sub-assembly, the optical signal irradiated from the LD 11 disadvantageously returns to itself because of being a little reflected on the surfaces of the lens 12, the filter 13, and the ferrule 14, instead of being transmitted them. The back-reflected optical signal exerts a bad influence on the oscillation of the LD 11, thereby increasing the noise in a system. This means that the performance of the system becomes deteriorated. To protect the returning back of the light to the LD 11, an optical isolator is generally used.
The optical isolator is a circuit device that is used for a circuit for transmitting microwave or optical wave, which can transmit the wave to one direction of a transmission line, but cannot transmit it to the opposite direction of the transmission line. Typically, this device utilizes a large Faraday rotation angle that a magnetic material has.
In general, the price of the optical isolator is expensive and increases in proportion to the effective aperture size of the isolator. This is an obstacle to the low-price policy of an optical sub-assembly. That is, the price for a commonly used isolator, whose effective aperture diameter is 1.5 mm, is expensively $70 or so. If a low-price isolator, whose value is approximately $20, has to be used, the isolator must be located as closer to the ferrule as possible in order to utilize a narrow range of a beam width.
However, in the case that the isolator is mounted closer to the ferrule, it is mechanically difficult to position the isolator closer to the ferrule because of the 45-degree-reflecting filter, and in addition, this structure deteriorates the performance of the receiver stage.
Therefore, it is necessary that the isolator be moved to the laser diode side. However, this enlarges the effective aperture size of the isolator because the beam width must be become wider, and so disturbs the low-price policy of the isolator.
In the meantime, a triplexer requires an analog optical receiving part in addition to the functions of the conventional diplexer, because it must distribute the analog and digital CATV signals. Such a triplexer covers three optical wavelength bands including an analog optical signal wavelength as well as the optical wavelengths assigned to the two digital signals.
Unlike the diplexer, used for the conventional bi-directional communication, which comprises a laser diode (LD) for an upstream signal and a digital receiver (PIN-TIA) for a downstream digital signal, the triplexer further comprises a part for separating the analog optical signal out of the digital signal, because the triplexer must cover a downstream analog optical signal distributed to the conventional diplexer.
The triplexer is generally divided by integration degree into an external WDM coupler type triplexer, a built-in WDM coupler type triplexer, and an integrated triplexer.
Among these, although it is predicted that the integrated triplexer becomes a mainstream in the long run, the separation type triplexer that a WDM coupler is externally attached or internally built can be partially used for a middle stage or a particular system. The separation type triplexer can be used when the integrated triplexer cannot be immediately adapted to the ready-established subscriber system or when the analog signal and the digital signal are processed by the separate circuits.
Since this triplexer covers three optical wavelength bands and satisfies the optical and electrical performances for the digital and analog signals, the design and manufacturing processes are more diverse and difficult.
FIG. 4 is an exploded perspective view showing a conventional integrated triplexer. It includes an optical fiber 21, an optical fiber collimating lens 22, an analog receiver 23, an analog receiver-collimating lens 24, a 45-degree-reflecting filter 25, a laser diode 26, a laser diode-collimating lens 27, a digital receiver 28, and a digital receiver-collimating lens 29.
The analog receiver 23 receives an analog optical signal. After the optical fiber collimating lens 22 converts the analog optical signal provided from the optical fiber 21 into a parallel beam, the analog receiver-collimating lens 24 delivers the parallel beam to the analog receiver 23.
The 45-degree-reflecting filter 25 separates the upstream and downstream signals at an angle of 45?, in which the optical isolation between the digital signals is approximately −20 dB. For the laser diode 26, an approximately 1310 nm FP laser diode or a DFB laser diode is used.
The laser diode-collimating lens 27 converts the light irradiated from the laser diode 26 into a parallel beam. Its effective focal length is approximately 1.5 mm, but this may be varied according to the design specifications.
The digital receiver 28, generally comprising InGaAs PIN-TIA, receives a digital optical signal. After the optical fiber collimating lens 22 converts the analog optical signal provided from the optical fiber 21 into a parallel beam, the digital receiver-collimating lens 29 delivers the parallel beam to the digital receiver 28. For the digital receiver-collimating lens 29, aspherical or spherical lens is used.
However, the conventional integrated triplexer optical sub-assembly has complicated structure and so its manufacturing cost increases, because many collimating lenses and mechanical elements are required for it. In addition, since the beam width is wider because of the collimating lenses, a filter size must be enlarged, thereby the external dimension increasing and manufacturing being difficult.